(a) Fields of the Invention
The present invention relates to methods for fabricating a semiconductor device using group III nitride, such as a field effect transistor, a bipolar transistor, or a light emitting element. In particular, the present invention relates to techniques to split a substrate into separate semiconductor elements.
(b) Description of Related Art
Group III nitride semiconductors made of GaN, AlN, InN, or a mixed crystal composed of these materials (hereinafter, referred simply to as nitride semiconductors) find applications in light emitting elements because they have wide band gap. Besides, they also find utilizations in developments of, for example, high-frequency, high-power electronic devices such as field effect transistors or bipolar transistors because they have high breakdown voltage, high electron saturation velocity and high electron mobility.
In fabricating semiconductor devices using these nitride semiconductors, it is difficult to produce single crystal wafers made of the nitride semiconductors. Thus, a nitride semiconductor crystal layer has been grown on a base substrate made of a material with a lattice constant and a thermal expansion coefficient different from those of the nitride semiconductor grown, such as a sapphire substrate and a SiC substrate. When a nitride semiconductor crystal layer is grown on a sapphire substrate or a SiC substrate, however, residual stress induced by lattice mismatch between the substrate and the nitride semiconductor crystal layer and accompanying residual strain are present and the stress and strain cause a problem such as bowing of the substrate. To overcome this problem, as one approach for removing the residual strain, a technique is developed in which the base substrate is irradiated with light from the back surface side thereof to form, at the interface between the substrate and the nitride semiconductor crystal layer, a thermally decomposed layer made by thermally decomposing the crystal layer (see Japanese Unexamined Patent Publication No. 2003-37286 and U.S. Pat. No. 6,071,795).
The conventional technique in which the base substrate is merely irradiated with light from the back surface side thereof to form a thermally decomposed layer at the interface between the substrate and the nitride semiconductor crystal layer, however, creates the following problems. Since the thermally decomposed layer is mainly composed of group III metal, it has a low melting point and high chemical reactivity. Thus, when a normal semiconductor fabrication step such as the step of heating a wafer or the step of exposing a wafer into a reactive gas atmosphere is conducted after the thermally decomposed layer formation step, the group III metal evaporates from the thermally decomposed layer to cause contamination. Furthermore, chemical reactions resulting from the normal fabrication step alter or deform the thermally decomposed layer itself to impair the uniformity of the layer within the wafer.
Moreover, since the entire back surface of a wafer is irradiated with light in the conventional technique, the thermally decomposed layer described above is created at the entire interface between the nitride semiconductor layer and the base substrate. As a result, the semiconductor layer cannot be kept fixed fully onto the base substrate, so that the semiconductor layer is displaced in a semiconductor device fabrication process. In this case, even though the thermally decomposed layer is fixed again in the course of a subsequent cooling of the substrate, the layer is melted again, for example, in the case where the substrate temperature is elevated above the room temperature in the process of semiconductor element fabrication on the substrate, resulting in displacement of the fabricated semiconductor element from the substrate. This is because the thermally decomposed layer is mainly composed of Ga, Al, In, or the like belonging to group III metal and thus has a low melting point.